1. Field of the Invention
The present invention relates to an in-vehicle white line recognition apparatus that recognizes a white line on a road for drive assistance.
2. Description of Related Art
An in-vehicle white line apparatus recognizes a white line on a road and is used in a lane departure warning apparatus to assist a driver in driving a vehicle (see Patent Document 1).
The lane departure warning apparatus includes an on-board camera for capturing an image of surroundings of the vehicle, an image processing unit for performing processing on the image, and a warning generation unit for generating a warning. The image processing unit detects or estimates a white line from the image captured by the on-board camera. The warning generation unit calculates a distance from the vehicle to a left side white line and a distance from the vehicle to a right side white line. When one of the distances is less than or equal to a threshold, i.e., when the vehicle approaches within the threshold from one of the distances, the warning generation unit gives a warning to a driver of the vehicle. In the following, the vehicle equipped with the lane departure warning apparatus is also called a subject vehicle.
A largest problem with the lane departure warning apparatus may be white line wrong detection caused by various noises. Noise sources include, for example, a dirty mark on a road surface, a paint on a road, a light reflected from a puddle etc. on a road surface, a shadow of the subject vehicle, a shadow of a vehicle in rear of the subject'vehicle, and the like.
Various techniques or methods to suppress theses noises and extract only a true white line have been proposed. For example, a method may include extracting white line candidates from an image captured by an on-board camera by using an image recognition technique, calculating certainty factors of respective white line candidates, and selecting a white line candidate having a largest certainty factor. In Patent Document 1, a certainty factor is calculated from brightness contrast between a white line candidate and its surroundings etc.
Patent Document 1: JP-2005-18148A
According to the method of calculating the certainty factor only from, the brightness contrast between a white line candidate and its surroundings, however, it is difficult to eliminate a possibility of wrong detection of an erased line trace (i.e., an white line that has been erased while a visible trace is being left) and the like in some cases. This will be more specifically described below.
Although a typical ratio of contrast between a white line and a road surface is approximately 400%, the ratio of contrast can be approximately 150% in some places. For this reason, when the ratio of contrast is not less than 150%, the certainty factor of the white line candidate is increased. However, a ratio of contrast between the erased line trace and the road surface can be approximately 160% in some cases. If the certainty factor of the erased line trace is increased, then the erased line trace is misidentified as a white line.